The present invention relates to a technique of performing cryptographic communication through a communication network such as the Internet.
When cryptographic communication is performed between communication terminals through a network, there is a method in which a common key for cryptographic communication is shared in advance between the communication terminals and the cryptographic communication is performed using the shared common key according to the following order. A communication terminal which initiates communication acquires a public key of a communication terminal of a communication counterpart. Then, the communication terminal which initiates the communication creates a common key to be used in the cryptographic communication with the communication terminal of the communication counterpart, encrypts the common key using the public key of the communication terminal of the communication counterpart, and transmits the encrypted common key to the communication terminal of the communication counterpart.
The communication terminal of the communication counterpart receives the common key encrypted with its public key from the communication terminal which initiates the communication, and decrypts the received encrypted common key by using private key of the communication terminal of the communication counterpart corresponding to the public key. Thus, the communication terminal which initiates the communication shares a common key to be used in the cryptographic communication with a communication terminal of communication counterparts.
In the above-described method, if the communication terminal which initiates the communication performs cryptographic communication with communication terminals of a plurality of communication counterparts, it is necessary to share the common key for cryptographic communication with the communication terminals of the plurality of communication counterparts, and accordingly, a process load of the communication terminal which initiates the communication may be increased. Therefore, Mark Baugher et al., “MSEC Group Key Management Architecture <draft-ietf-msec-gkmarch-07.txt>”, IETF (Internet Engineering Task Force), P3-13 (hereinafter, referred to as Non-Patent Document 1) discloses a technique in which a server, which distributes a common key for cryptographic communication to a communication terminal which initiates communication and communication terminals of communication counterparts, is provided on a network and the communication terminal which initiates the communication and the communication terminals of communication counterparts perform the cryptographic communication by using the common key distributed by the server. In the technique disclosed in Non-Patent Document 1, the server creates the common key for cryptographic communication between the communication terminal which initiates the communication and the communication terminals of communication counterparts and distributes the created common key to the communication terminal which initiates the communication and the communication terminals of communication counterparts, thereby reducing the process load of the communication terminal which initiates the communication, which is created when the communication terminal which initiates the communication creates the common key.